New Study Reveals the Atmospheric Footprint of the Global Warming Hiatus

The increasing rate of the global mean surface temperature was reduced from 1998 to 2013, known as the global warming hiatus or pause. Great efforts have been devoted to the understanding of the cause. The proposed mechanisms include the internal variability of the coupled ocean-atmosphere system, the ocean heat uptake and redistribution, among many others. However, the atmospheric footprint of the recent warming hiatus has been less concerned. Both the dynamical and physical processes remain unclear.

In a recent paper published in Scientific Report, LIU Bo and ZHOU Tianjun from the Institute of Atmospheric Physics, Chinese Academy of Sciences have investigated the atmospheric anomalous features during the global warming hiatus period (1998-2013). They show evidences that the global mean tropospheric temperature also experienced a hiatus or pause (Fig. 1). To understand the physical processes that dominate the warming hiatus, they decomposed the total temperature trends into components due to processes related to surface albedo, water vapor, cloud, surface turbulent fluxes and atmospheric dynamics. The results demonstrated that the hiatus of near surface temperature warming trend is dominated by the decreasing surface latent heat flux compared with the preceding warming period, while the hiatus of upper tropospheric temperature is dominated by the cloud-related processes. Further analysis indicated that atmospheric dynamics are coupled with surface turbulent heat fluxes over lower troposphere and coupled with cloud processes over upper troposphere.

Figure 1. (a) Global mean temperature anomalies from 1950 to 2015 and (b) linear trends of global mean temperature for near surface (i.e. the lowest atmospheric layer), and the vertical average of the whole (surface to 100hPa), lower (surface to 500hPa), and upper troposphere (500hPa to 100hPa). Red (black) bars are for the warming period. Blue(white) bars are for the hiatus period. (Liu and Zhou, 2017)

As to why the surface latent heat flux, atmospheric dynamics and cloud-related processes showed such large differences between 1983-1998 and 1998-2013, LIU, the first author of the paper, explained, “They are dominated by the Hadley Circulation and Walker Circulation changes associated with the phase transition of Interdecadal Pacific Oscillation (IPO).” According to LIU, the IPO is a robust, recurring pattern of sea surface temperature anomalies at decadal time scale. During a positive phase of IPO, the west Pacific and the mid-latitude North Pacific becomes cooler and the tropical eastern ocean warms, while during a negative phase, the opposite pattern occurs. The IPO has shifted from the positive phase to negative phase since 1998/1999, and this transition has led to the weakening of both Hadley Circulation and Walker Circulation, which served as a hub linking the three processes mentioned above.

“Though the heat capacity of the atmosphere is nearly negligible compared with the ocean”, said ZHOU, the corresponding author of the paper, “understanding the atmospheric footprint is essential to gain a full picture of how internal climate variability such as IPO affects the global climate from the surface to the troposphere. The new findings also provide useful observational metrics for gauging climate model experiments that are designed to understand the mechanism of global warming hiatus”.

The post-2002 global surface warming slowdown caused by the subtropical Southern Ocean heating accelerationThe warming rate of global mean surface temperature slowed down during 1998–2012. Previous studies pointed out role of increasing ocean heat uptake during this global warming slowdown, but its mechanism remains under discussion. Our numerical simulations, in which wind stress anomaly in the equatorial Pacific is imposed from reanalysis data, suggest that subsurface warming in the equatorial Pacific took place during initial phase of the global warming slowdown (1998–2002), as previously reported. It is newly clarified that the Ekman transport from tropics to subtropics is enhanced during the later phase of the slowdown (after 2002) and enhanced subtropical Ekman downwelling causes accelerated heat storage below depth of 700 m in the subtropical Southern Ocean, leading to the post-2002 global warming slowdown. Observational data of ocean temperature also support this scenario. This study provides clear evidence that deeper parts of the Southern Ocean play a critical role in the post-2002 warming slowdown.

This scientific “truth” was drilled into me, a young geology undergrad in Calgary, by esteemed professors in basic courses at the beginning of the 1980s. In the 1970s the media were abuzz with global-cooling scares. Cooling was supposedly a scientific fact. Thankfully, the old fears of an impending new ice age have so far proved unfounded. But now, global warming has replaced the global-cooling craze. This article is not, by any means, a final word. I am neither a climatolo- gist nor a logician. My purpose is to encourage the readers to explore scientific logic, to always be skeptical, to question the methods and the motives, and to always be ready to wonder and be surprised.

Empiricism

Natural science is empirical. Empiricism says knowledge is derived from what we can sense or observe. Knowledge is gained by passive observation of natural occurrences or by active, preferably controlled, experiments. Epistemology is the study of human knowledge. It deals with how we know things.Much philosophical ink has been spilled on these subjects over the past several millennia. Too much of that ink flowed uselessly, or it is irrelevant to the discussion at hand. Only a few key points are summarized below.Karl Popper (1950, 1968) was probably the past century’s foremost empirical philosopher. He taught that a legitimate scientific hypoth- esis must be falsifiable, i.e., capable of being disproved by subsequent observations or experiments. If a “theory rules out certain possible occurrences, … it will be falsified if these possible occurrences do in fact occur”.